In Avatar: the Last Airbender, it is up to the titular Avatar Aang to master the four elements and end the Fire Nation's war of conquest. However it's one of the show's supporting characrers, Uncle Iroh, who truly symbolizes the positive messages of this surprisingly deep animated series.
Unity Of All
One of the many uplifting and inspiring themes of Avatar: the Last Airbender is the theme of unity, that separation is a mere illusion. This is a core tenant of real-life Buddhism: that all people, animals and other entities are different physical manifestations of the same whole. Shakespeare once said "one touch of nature makes the whole world kin," and this is true. Nations and laws and geographic boundaries are the creations of humanity, not the universe, and Iroh sees past them when few other people do.
In many ways, Iroh brings together the nature of the four elements better than any non-Avatar person could. For example, he invented the resourceful technique of redirecting lightning, which is derived not from firebending forms, but from waterbending. This allows the bender to channel their opponent's lightning, channel it through their body and emit it from one hand, like a waterbender does with water and ice.
Iroh is the least prejudiced person in the show, and he never shared his family's sense of Fire Nation supremacy. One century ago, Fire Lord Sozin went as far as to say that the whole world should be Fire Nation, and his grandson Ozai plotted to rule the world as the so-called Phoenix King. Ozai's brother Iroh, however, laid down his arms after the failed siege of Ba Sing Se and wandered the globe with his nephew Zuko. Iroh saw the people of the Earth Kingdom up close, and learned to appreciate their way of life.By the time he comfortably settled down in Ba Sing Se, there was no trace of his Fire Nation origins... excepting his infamous tea-heating.
Forgoing nationalist allegiances, Iroh is a dedicated member of the White Lotus order. The White Lotus draws members from all of the nations to form a flagless corps that is loyal only to the Avatar.
Forgiveness
Iroh is hostile to his Fire Lord brother and has a poor opinion of Azula, but he is the perfect father figure for his nephew Zuko. Even after Zuko turned on Iroh to be accepted back into the Fire Nation, Iroh didn't hold a grudge against him. Instead, Iroh waited patiently for his time to act.
This led to one of Avatar's most emotional scenes, when Zuko came face-to-face with his uncle and was suddenly embraced in a loving hug. Zuko didn't think he was worth forgiving, but Iroh did, and Zuko soon agreed with his uncle's opinion.
Iroh gave aid to Team Avatar, and Aang and the others came to accept him as a friend. They followed Iroh's example of forgiveness and in turn forgive both him and Zuko for their roles in the Fire Nation's brutality.
Paradoxically, Iroh struggles to forgive himself, as made clear in his heartbreaking story in "Tales of Ba Sing Se." He still cannot accept his role in his son Lu Ten's death. This outstanding issue helps add depth to Iroh's character and makes him more compelling, as even the best of us struggle to always live up to our ideals.